The present invention relates to method of fabricating a prestressed cast iron vessel. It particularly concerns a method of fabricating a vessel of large volume capable of withstanding large internal pressures.
Industrial vessels have been built for years by forming metal sections and welding them together in the desired shape to form steel vessels. The welding becomes increasingly time consuming and expensive; however, when larger sized vessels are designed requiring the steel wall thickness to exceed about six inches. For this reason, prestressed concrete vessels have been used in recent years to overcome the problems associated with constructing large steel vessels having substantial wall thickness. Even more recently, cast iron has been determined to be a valuable material with attractive properties that can be used in much the same way as concrete to construct large prestressed cast vessels.
The impetus for consideration of cast iron as a structural material is the superior compressive strength and temperature capability of cast iron as compared to concrete, both of which appear to offer the potential for improvements in both safety and economics associated with large vessels. Conceptual design studies of prestressed cast iron vessels have shown that using machined cast iron blocks in lieu of concrete is feasible and that capital cost and construction time can be reduced.
The problem in constructing prestressed cast iron vessels is fundamentally a fit-up problem. The cast iron body of the vessel must be made from numerous segments that are machined individually and stacked appropriately to form the vessel body. To prevent leakage through the wall of such a structure, a ductile metal liner must be used inside the vessel body. The principal problem with such an arrangement is building the liner and the cast iron segments with sufficient dimensional control that good contact results when the body segments are grouped around a liner and prestressed. If too much clearace is permitted between the liner and prestressed cast iron vessel body, the liner may tear during hydrotesting. If too much interference is present, the liner may buckle during prestressing. The key is to find a practical method of fabricating a prestressed cast iron vessel that will produce acceptable contact conditions between the cast iron body and the internal liner.
Previously, fabrication methods for constructing a prestressed cast iron vessel involved construction of the liner as a shell followed by stacking the cast iron body segments around the shell, followed by wrapping the cast iron body segments with the circumferential prestressed members and finally filling the void space between the liner and cast iron body segments with a strong filler material such as grout. The foregoing outlined method presents several major problem areas such as, the circumferential prestressing which is required at various elevations on the vessel is difficult because it is slow and dangerous, particularly where tall vessels are involved. Additionally, filler material such as grout placed into the crevices between the liner and cast iron body segments is difficult to inspect and may not be strong enough to function depending upon the pressures encountered. Use of a filler such as grout establishes undesirable temperature gradients between the liner and cast iron body segments due to the presence of the grout resulting in unnecessary and perhaps dangerous thermal stresses. Finally, the filler material, regardless of its initial strength, may fatique, crack and spall during operation, again resulting in an undesirable situation that is difficult to detect and to correct.